He's saying that since the S&P is at a higher multiple, you'd expect returns to diminish since presumably its too high to keep growing quickly (and, potentially

He's saying that since the S&P is at a higher multiple, you'd expect returns to diminish since presumably its too high to keep growing quickly (and, potentially, turn into a bear/down market). So by performing at average, its actually out-performing what you'd expect given the conditions. (Of course that's the theory, clearly the market isn't solely governed by its multiple and risk level)